Oil shale is a potentially significant source of liquid hydrocarbons. Oil shale is immature petroleum source rock that has not been buried deep enough to generate significant quantities of liquid hydrocarbons. The recovery process often involves heating the oil shale to a temperature in the general range of 300 to 500° C. (degrees Celsius) to convert the native organic matter, primarily kerogen, to oil and gas. The time for this conversion varies with temperature, ranging from, for example, years at 300° C. to minutes at 500° C. Underground (i.e., in situ) processing is typically done at temperatures below 400° C., and aboveground processing (i.e., mining and retorting in a vessel) is typically done at temperatures above 400° C.
There are many variations of in-situ processing. Some involve creating permeability by explosive nibbling, and others involve waiting for thermal conductivity to distribute the heat through the oil shale. Some involve injecting a hot fluid into the formation, and others allow heat to dissipate from a passive heater into the formation. Passive heaters may include, for example, electric heaters, downhole burners, or pipes with recirculating hot fluids. Some variations of the passive heater concept use refluxing oil within the retort to speed the dissipation of the heat from the passive heater into the formation.
Earlier concepts for using a hot recirculating fluid to heat oil shale include using heat transfer fluids such as steam, molten salt, simple gases, Dowtherm® A and Syltherm®, available from the Dow Corning Corporation of Midland, Mich., U.S.A., and Therminol® VP-1, available from the Monsanto Chemical Company, of St. Louis, Mo., U.S.A. Generally, transfer fluids are selected to maximize the amount of heat delivered while the minimizing the amount of pumping costs and parasitic heat loss.
Fixed gases such as nitrogen and carbon dioxide have low heat capacities and, thus, require high temperatures and often significant pumping costs. Synthetic fluids such as Dowtherm® A, Syltherm®, and Therminol® VP-1 have higher heat capacities but also have a maximum operating temperature only slightly above retorting temperature. Consequently, only a small fraction of heat can be delivered each cycle, which may result in significant pumping costs. Steam has the disadvantage that it has to be used at very high pressures to prevent most of its heat from being delivered at sub-retorting temperatures via condensation. Molten salts are corrosive and have operational issues such as solidifying during operational upsets. Other gaseous fluids such as hexafluoroethane have attractive thermodynamic properties but are quite expensive, and some are potent greenhouse gases.